Hitherto, there has been known cellulosic bioethanol, which is produced from biomass materials containing cellulose or hemicellulose.
There has also been known a method for producing a saccharide (e.g., glucose) from cellulosic biomass materials containing cellulose or hemicellulose (i.e., a saccharification technique). In the method, the cellulosic biomass materials are hydrolyzed with sulfuric acid. The method involves problems such as corrosion of a reactor and treatment of wastewater. In another known saccharification method, cellulosic biomass materials are saccharified in the presence of a solid acid catalyst formed of a support (e.g., carbon or zeolite) on which sulfa groups are present. This method also has problems of a considerably slow reaction rate due to solid reaction and difficulty in separation of the solid acid catalyst from the unreacted residue. Furthermore, in the above methods, difficulty is encountered in controlling hydrolysis. When the hydrolysis reaction proceeds excessively, the formed saccharide decomposes, to thereby lower the yield of the saccharide of interest.
Also, enzymatic saccharification is known to be performed in the presence of an enzyme (see Patent Document 1). Such a method includes a hydrothermal step of treating a raw material with pressurized hot water, a mechanical crushing step of the hydrothermal treatment product, and a saccharifying step of saccharifying the mechanically crushed product. However, according to the method, enzymatic saccharification rate is low, whereby the produced saccharified liquid concentration is not always sufficient, which problematic.
In order to solve the problem, there has been proposed an improved method which can promote enzymatic reaction more efficiently. In the method, the enzyme is immobilized into the meso-porous of a meso-porous silica in the reaction, whereby the enzyme is caused to be present in the reaction system at a higher concentration, as compared with the case in which the enzyme is dissolved in the reaction system (see Patent Document 2). However, this method involves some problems. Specifically, the method requires an additional step of causing the enzyme to be adsorbed into the support for immobilization, and the thus-immobilized enzyme may attain a reduced reaction efficiency of only about 40 to about 50%, as compared with the case of the same enzyme in a non-immobilized state. Furthermore, difficulty is encountered in separating the enzyme-fixed support from the unreacted residue, due to the solid-solid phase reaction.
Also known is a powder form immobilized enzyme prepared by mixing an enzyme with silica sol, gelling the silica to a corresponding silica gel, and crushing the product (see Patent Documents 3 and 4). Even when such a powder-form immobilized enzyme is employed, the enzyme can be recovered, but the reaction efficiency is poor in another known method, dietary fiber containing cellulose is hydrolyzed with a mixture of an enzyme and a silica powder having a particle size of 0.5 μm to 100 μm. However, the effect of mixing the silica powder cannot be definitely proven, and difficulty is encountered in separating the suspended silica powder from the unreacted residue (see Patent Document 5).